The part of the body on which one sits; the buttocks; specifically (Anat.), the anus.

the fleshy part of the human body that you sit on; "he deserves a good kick in the butt"; "are you going to sit on your fanny and do nothing?"

the fundamental assumptions from which something is begun or developed or calculated or explained; "the whole argument rested on a basis of conjecture"

lowest support of a structure; "it was built on a base of solid rock"; "he stood at the foot of the tower"

Foundation.